Autonomous apparatuses, such as robotic vacuum cleaners and other self-propelled devices, are well known in the prior art as, for example, disclosed in International Patent Applications WO 97/41451 (U.S. Pat. No. 5,935,179) and WO 00/38028. These autonomous apparatuses, generally, have a main body, supported on or by a number of motor-driven wheels or rollers and include means for surface treatment, such as a rotating brush in combination with a vacuum cleaning device. Typically, they are provided with systems for guidance and navigation, together with a sensing system for obstacle detection. The sensing system generally sweeps around the horizon in a manner similar, for example, to a ship's radar. The autonomous devices usually include microprocessors, together with appropriate software, for controlling the functioning of the devices. Normally, a microprocessor receives input data from the motor driven wheels and the sensing system for the purpose of determining the position of the device and the locations of wall limitations and potential obstacles. This input data is then used as the basis for navigating the autonomous device so that it will be able, for instance, to perform a cleaning function or other surface treatment function on a field of operation according to some predetermined strategy while at the same time avoiding collisions with any obstacles or barriers such as walls, tables, bookcases or the like.
Typically, an autonomous apparatus orients itself at any time within a field of operation defined by a map that is generated on the basis of information obtained from a sensing system. Furthermore, the map, typically, is updated by the autonomous apparatus during its movements. It is important that as accurate a map as possible be generated. It is also important that the exact position and orientation of the apparatus within the field of operation defined by the map be determined in order for the apparatus to satisfactorily perform its functions. Although several types of systems for the self-determination by an autonomous apparatus of its position have been developed, systems having distance sensors that take their input information from the rotation of the wheels on which the autonomous apparatus is transported, have proven to be most cost-effective.
Cost-effective, internal, position-determination systems, usually, comprise wheel sensors that register the number of times the wheels have rotated. In order to determine the distance the apparatus has traveled since a previous registration, the number of wheel rotations is applied to the diameters of the wheels. This distance calculation can be made with reasonably good accuracy when the surface of the field of operation is not too soft and the effective wheel diameter can be assumed to be constant. However, when the surface is slippery or soft, the apparatus may skid and/or the wheels may spin. In those instances, the calculated distance traveled, based on the number of wheel rotations, is incorrect. In order to correct for such circumstances, a position calibration method has to be carried out. Such a method is disclosed in U.S. Pat. No. 5,794,166.
The method disclosed in U.S. Pat. No. 5,794,166 includes predicting the cumulative overall slippage of the apparatus that occurs when the apparatus travels from a starting point to a destination point along an imaginary path. The creation of the imaginary path includes a first step of rotating the apparatus at the starting point so that the apparatus is directed toward the point of destination. The apparatus is assumed to then travel forward from the staring point to the destination point. Finally, a rotation of the apparatus at the destination point to align it in a predetermined position is assumed to occur. An overall slippage component for each wheel is calculated, based on at least one distance-dependent slippage factor.
To perform the method of U.S. Pat. No. 5,794,166, the track width of the autonomous apparatus must be known. The track width is the distance that separates the effective contact points between the two driving wheels and the surface over which the apparatus operates. The track width is also utilized for calculating the angles through which the autonomous apparatus would be required to turn along the imaginary path.
A difficulty with the method disclosed in U.S. Pat. No. 5,794,166 is that the actual track width of the autonomous apparatus will vary, depending on the type of surface over which the apparatus is to travel and the nature and direction of any turns the apparatus makes. This is because the locations of the effective contact points between the wheels and the surface will vary, so as to vary the actual track width, and because the suspensions of the wheels are not totally rigid. As the effective contact points are displaced during turning of the apparatus, the actual track width will change, and the slippage calculation made in accordance with the method disclosed in U.S. Pat. No. 5,794,166 will prove to be erroneous.